PURPOSE: To allow the dual system to return to its original dual operation smoothly without copying any file when a faulty CPU recovers by providing the dual system with a substitute data bus for the case of a CPU fault.

CONSTITUTION: When the 1st CPU10 is troubled, the dual system reports that to the 2nd CPU20. Consequently, the CPU20 brings a file storage device 11 connected to the CPU10 so far under its control by using the alternative data bus 23. The updating operation of files after said point of time is carried out for file storage devices 21 and 11 to make the contents of both file storage devices coincident. Then, when the repair of the CPU10 is completed and processing is placed in readiness, the CPU10 reports that to the CPU20. Consequently, the CPU20 reconnects the device 11 from the CPU20 to the CPU10 and the system enters its original dual operation state and restarts data processing smoothly without copying any file.